U.S. patent application number 12/497367 was filed with the patent office on 2011-01-06 for acoustic crystal sonoluminescent cavitation devices and ir/thz sources.
This patent application is currently assigned to Raytheon Company. Invention is credited to Delmar L. Barker, William R. Owens.
Application Number | 20110001063 12/497367 |
Document ID | / |
Family ID | 43412113 |
Filed Date | 2011-01-06 |
United States Patent
Application |
20110001063 |
Kind Code |
A1 |
Barker; Delmar L. ; et
al. |
January 6, 2011 |
ACOUSTIC CRYSTAL SONOLUMINESCENT CAVITATION DEVICES AND IR/THZ
SOURCES
Abstract
An acoustic crystal structure includes defect cavities that
concentrate the driving pressure from applied sound waves into the
cavities to cavitate gas bubbles in a liquid to produce
sonoluminescence. This device may be used to study sonoluminescence
or cavitation or to perform sonochemistry, nuclear fusion etc. in
the cavities. A waveguide may be operatively coupled to the
acoustic crystal to extract, collect and route a band of
electromagnetic (EM) radiation around a specified source wavelength
to an output port for emission by an antenna to provide an EM
source. The waveguide may, for example, be a photonic crystal
defect waveguide, a photonic crystal optical fiber or Sommerfeld
waveguide. The marriage of the sonoluminescence phenomena with an
acoustic crystal and embedded waveguide provides for an efficient
source of narrow or broad band IR or THz radiation
Inventors: |
Barker; Delmar L.; (Tucson,
AZ) ; Owens; William R.; (Tucson, AZ) |
Correspondence
Address: |
Eric A. Gifford (Raytheon Company)
11770 E. Calle del Valle
Tucson
AZ
85749
US
|
Assignee: |
Raytheon Company
|
Family ID: |
43412113 |
Appl. No.: |
12/497367 |
Filed: |
July 2, 2009 |
Current U.S.
Class: |
250/493.1 |
Current CPC
Class: |
G21B 3/00 20130101; Y02E
30/10 20130101; Y02E 30/18 20130101 |
Class at
Publication: |
250/493.1 |
International
Class: |
G21G 4/00 20060101
G21G004/00 |
Claims
1. A sonoluminescent cavitation device, comprising: an acoustic
crystal including first and second materials having different
acoustic indices, said materials arranged in a periodic array that
provides local contrast modulation of the acoustic index in at
least one dimension to define a band gap in the acoustic or
ultrasonic transmission spectra; at least one defect cavity in the
periodic array that creates a resonance in the band gap; a liquid
in said defect cavity; a gas dissolved in the liquid; and a source
of sound waves that are operatively coupled to the periodic array,
said sound waves concentrated in said at least one defect cavity to
cause the bubble to expand and collapse producing a pulse of
black-body radiation.
2. The sonoluminescent cavitation device of claim 1, wherein the
contrast modulation is at least 1.3 for a 1D periodic array, 1.6
for a 2D periodic array and 2 for a 3D periodic array.
3. The sonoluminescent cavitation device of claim 1, wherein the
band gap lies in the acoustic spectra from approximately 20 kHz to
500 kHz.
4. The sonoluminescent cavitation device of claim 3, wherein the
periodic spacing of the first and second materials in the periodic
array is between 100 microns and 6 cm.
5. The sonoluminescent cavitation device of claim 1, further
comprising a plurality of said defect cavities that contain a gas
dissolved in a liquid, said sound waves causing bubbles to expand
and collapse in said plurality of said defect cavities producing
respective pulses of black-body radiation.
6. The sonoluminescent cavitation device of claim 1, wherein said
defect cavities are cylindrical, a plurality of gas bubbles forming
along a long axis of the cylinder.
7. The sonoluminescent cavitation device of claim 1, wherein said
sound waves are characterized by a dominant frequency that lies
within the band gap.
8. The sonoluminescent cavitation device of claim 7, wherein the
dominant frequency is approximately equal to the resonance
frequency of the defect cavity.
9. The sonoluminescent cavitation device of claim 1, wherein said
source of sound waves comprises a single piezo-electric
transducer.
10. The sonoluminescent cavitation device of claim 1, wherein said
pulse of black-body radiation induces or enhances a desired
chemical reaction in the surrounding liquid in the defect
cavity.
11. A sonoluminescent source of electromagnetic radiation,
comprising: an acoustic crystal including first and second
materials having difference acoustic indices, said materials
arranged in a periodic array that provides local contrast
modulation of the acoustic index in at least one dimension to
define a band gap in the acoustic or ultrasonic transmission
spectra; at least one defect cavity in the periodic array that
creates a resonance in the band gap; a liquid in said defect
cavity; an inert gas dissolved in the liquid; a source of sound
waves that are operatively coupled to the periodic array, said
sound waves concentrated in said at least one defect cavity to
cause the bubble to expand and collapse producing a pulse of
black-body radiation; and a waveguide operatively coupled to said
acoustic crystal, said waveguide tuned to a specified source
wavelength to extract a band of the black-body radiation around the
source wavelength emitted from said at least one defect cavity,
collect the radiation in said band and route the radiation to an
output port.
12. The sonoluminescent device of claim 11, wherein the band gap
lies in the acoustic spectrum from approximately 20 kHz to
approximately 500 kHz and the band lies in the THz spectrum from
approximately 300 GHz to approximately 4 THz or the infrared (IR)
spectrum from approximately 1 micron to approximately 30
microns.
13. The sonoluminescent device of claim 11, wherein the band lies
in the THz spectrum and has a band width between approximately 30
GHz and approximately 300 GHz.
14. The sonoluminescent device of claim 11, wherein the band lies
in the IR spectrum and has a band width between approximately 0.1
micron and approximately 3 microns.
15. The sonoluminescent device of claim 11, further comprising a
plurality of said defect cavities that contain an inert gas
dissolved in a liquid, said sound waves causing bubbles to expand
and collapse in said plurality of said defect cavities producing
respective pulses of black-body radiation, said waveguide
extracting radiation in said band from the plurality of defect
cavities, collecting and routing the radiation to the output
port.
16. The sonoluminescent device of claim 11, wherein the physical
size of the waveguide is less than one wavelength of the sound
waves.
17. The sonoluminescent device of claim 11, further comprising a
second waveguide operatively coupled to said acoustic crystal, said
second waveguide tuned to a different second specified source
wavelength to extract a different second band of the black-body
radiation around the second source wavelength from said at least
one defect cavity, collect the radiation in said band and route the
radiation to a second output port.
18. The sonoluminescent device of claim 11, wherein the waveguide
comprises: a photonic crystal including third and fourth materials
having difference refractive indices, said materials arranged in a
periodic array that provides local contrast modulation of the
refractive index in at least one dimension to define a band gap in
the electromagnetic spectrum, said photonic crystal having a period
at least 10.times. smaller than the period of the acoustic crystal;
and at least one defect in the periodic array of the photonic
crystal tuned to resonate at the specified source wavelength and
positioned to couple to the at least one defect cavity in the
acoustic crystal, said at least one defect extracting radiation in
a band around the source wavelength from said at least one defect
cavity, collecting the radiation in said band and routing the
radiation to the output port.
19. The sonoluminescent device of claim 11, wherein the waveguide
comprises a linear conductor proximate each of said at least one
defect cavities, said linear conductor tuned to couple radiation in
a band around the specified source wavelength emitted from the
defect cavities in said pulse.
20. The sonoluminescent device of claim 11, wherein the waveguide
comprises a photonic crystal optical fiber.
21. The sonoluminescent device of claim 11, wherein the sound waves
produce a sequence of pulses of black-body radiations, a like
sequence of pulses of radiation in said band around the specified
source wavelength routed to the output port.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to sonoluminescent cavitation devices
and to their use as sources of infrared (IR) or terahertz (THz)
radiation.
[0003] 2. Description of the Related Art
[0004] Sonoluminescence is a well-known phenomenon discovered in
the 1930's in which light is generated when a liquid is cavitated.
Although a variety of techniques for cavitating the liquid are
known (e.g., spark discharge, laser pulse, flowing the liquid
through a Venturi tube), one of the most common techniques is
through the application of high intensity sound waves.
[0005] In essence, as shown in FIGS. 1 and 2 the cavitation process
consists of three stages that are driven by the application of high
intensity sound waves 8; bubble formation 10, growth 12 and
subsequent collapse 14 that emits a flash or pulse of light 16. The
bubble or bubbles cavitated during this process absorb the applied
energy, for example sound energy, and then release the energy in
the form of light emission during an extremely brief period of
time. The intensity of the generated light depends on a variety of
factors including the physical properties of the liquid (e.g.,
density, surface tension, vapor pressure, chemical structure,
temperature, hydrostatic pressure, etc.) and the applied energy
(e.g., sound wave amplitude, sound wave frequency, etc.). In
general, the negative driving pressure causes the bubble to expand;
when the driving pressure changes sign, the bubble collapses,
resulting in a short pulse of light. The bubbles may grow and
collapse with each cycle of the sound wave as illustrated or may
grow over multiple cycles before collapsing.
[0006] Although it is generally recognized that during the collapse
of a cavitating bubble extremely high temperature plasmas are
developed, leading to the observed sonoluminescence effect, many
aspects of the phenomena have not yet been characterized. As such,
the phenomena is at the heart of a considerable amount of research
as scientists attempt to not only completely characterize the
phenomena (e.g., effects of pressure on the cavitating medium), but
also its many applications (e.g., sonochemistry, chemical
detoxification, ultrasonic cleaning, nuclear fusion, generation of
nano-particles etc.).
[0007] In a typical cavitation system, for example as shown by Dan
et al. in an article entitled Ambient Pressure Effect on
Single-Bubble Sonoluminescence (vol. 83, no. 9 of Physical Review
Letters), the cavitation chamber is a simple glass flask that is
filled or semi-filled with cavitation liquid. A spherical flask is
also disclosed in U.S. Pat. No. 5,659,173. The specification of
this patent discloses using flasks of Pyrex..RTM., Kontes..RTM.,
and glass with sizes ranging from 10 milliliters to 5 liters. The
drivers as well as a microphone piezoelectric were epoxied to the
exterior surface of the chamber.
[0008] In some instances, more elaborate chambers are employed in
the cavitation system. For example, U.S. Pat. No. 4,333,796
discloses a cavitation chamber designed for use with a liquid
metal. As disclosed, the chamber is generally cylindrical and
comprised of a refractory metal such as tungsten, titanium,
molybdenum, rhenium or some alloy thereof. Surrounding the
cavitation chamber is a housing which is purportedly used as a
neutron and tritium shield. Projecting through both the outer
housing and the cavitation chamber walls are a number of acoustic
horns, each of the acoustic horns being coupled to a transducer
which supplies the mechanical energy to the associated horn. The
specification discloses that the horns, through the use of flanges,
are secured to the chamber/housing walls in such a way as to
provide a seal and that the transducers are mounted to the outer
ends of the horns.
[0009] A tube-shaped cavitation system is disclosed in U.S. Pat.
No. 5,658,534, the tube fabricated from stainless steel. Multiple
ultrasonic transducers are attached to the cavitation tube, each
transducer being fixed to a cylindrical half-wavelength coupler by
a stud, the coupler being clamped within a stainless steel collar
welded to the outside of the sonochemical tube. The collars allow
circulation of oil through the collar and an external heat
exchanger.
[0010] Another tube-shaped cavitation system is disclosed in U.S.
Pat. No. 6,361,747. In this cavitation system the acoustic
cavitation reactor is comprised of a flexible tube. The liquid to
be treated circulates through the tube. Electroacoustic transducers
are radially and uniformly distributed around the tube, each of the
electroacoustic transducers having a prismatic bar shape. A film of
lubricant is interposed between the transducer heads and the wall
of the tube to help couple the acoustic energy into the tube.
[0011] U.S. Pat. No. 5,858,104 discloses a shock wave chamber
partially filled with a liquid. The remaining portion of the
chamber is filled with gas which can be pressurized by a connected
pressure source. Acoustic transducers are used to position an
object within the chamber while another transducer delivers a
compressional acoustic shock wave into the liquid. A flexible
membrane separating the liquid from the gas reflects the
compressional shock wave as a dilation wave focused on the location
of the object about which a bubble is formed.
[0012] PCT application Ser. No. US02/16761 (WO/2002/097823
published Dec. 5, 2002) discloses a nuclear fusion reactor in which
at least a portion of the liquid within the reactor is placed into
a state of tension, this state of tension being less than the
cavitation threshold of the liquid. The liquid preferably includes
enriched deuterium or titium, the inventors citing deuterated
acetone as an exemplary liquid. In at least one disclosed
embodiment, acoustic waves are used to pretension the liquid. In
order to minimize the effects of gas cushioning during bubble
implosion, the liquid is degassed prior to tensioning. A resonant
cavity is formed within the chamber using upper and lower pistons,
the pistons preferably fabricated from glass. The upper and lower
pistons are smaller than the inside diameter of the chamber, thus
allowing cavitation liquid to pass by the pistons. In a preferred
embodiment, the upper piston is flexibly anchored to the chamber
using wire anchors while the lower piston is rigidly anchored to
the chamber.
[0013] U.S. Pat No. 7,510,322 discloses a cavitation chamber
separated into three volumes by a pair of gas-tight and
liquid-tight seals, each seal formed by the combination of a rigid
acoustic reflector and a flexible member, is provided. During
chamber operation, only one of the three volumes contains
cavitation liquid, the other two chamber volumes remaining devoid
of cavitation liquid. The cavitation system also includes a
cavitation liquid reservoir coupled to the cavitation chamber by a
conduit, a valve allowing the cavitation chamber to be isolated
from the cavitation liquid reservoir. A second conduit couples the
two unfilled chamber volumes to a region above the liquid free
surface within the cavitation liquid reservoir. A second valve
allows the two unfilled chamber volumes to either be coupled to the
cavitation liquid reservoir by the second conduit, or be coupled to
a third conduit, the third conduit leading either to the ambient
atmosphere or to a high pressure gas source. The cavitation system
also includes at least one acoustic driver.
SUMMARY OF THE INVENTION
[0014] The following is a summary of the invention in order to
provide a basic understanding of some aspects of the invention.
This summary is not intended to identify key or critical elements
of the invention or to delineate the scope of the invention. Its
sole purpose is to present some concepts of the invention in a
simplified form as a prelude to the more detailed description and
the defining claims that are presented later.
[0015] The present invention uses an acoustic crystal structure to
enhance sonoluminescence. Defect cavities in the crystal structure
concentrate the driving pressure from applied sound waves into the
cavity to cavitate gas bubbles in a liquid to produce
sonoluminescence. A waveguide may be coupled to the acoustic
crystal to extract, collect and route a band of electromagnetic
(EM) radiation around a specified source wavelength to an output
port for emission by an antenna to provide an EM source. The
marriage of the sonoluminescence phenomena with an acoustic crystal
provides for an efficient source of narrow or broad band IR or THz
radiation
[0016] In an embodiment, a sonoluminescent cavitation device
comprises an acoustic crystal including first and second materials
having difference acoustic indices. The materials are arranged in a
periodic array that provides local contrast modulation of the
acoustic index in at least one dimension to define a band gap in
the acoustic or ultrasonic transmission spectra. At least one
defect cavity in the periodic array creates a resonance in the band
gap. A gas is dissolved in a liquid in the at least one defect
cavity. A source of sound waves is operatively coupled to the
periodic array. The sound waves are concentrated in the at least
one defect cavity to cause the bubble to expand and collapse
(cavitate) producing a pulse of black-body radiation (e.g.
sonoluminescence). This sonoluminescent device may be used to
research the sonoluminescent phenomena or study cavitation or as a
cavity to perform sonochemistry, nuclear fusion, create
nano-particles, chemical detoxification, ultrasonic cleaning
etc.
[0017] In an embodiment, a sonoluminescent EM source comprises a
waveguide operatively coupled an acoustic crystal sonoluminescent
cavitation device to extract, collect and route a band of
electromagnetic (EM) radiation around a specified source wavelength
to an output port for emission by an antenna to provide an EM
source. Sonoluminescence in the defect cavity produces very high
temperatures, hence elevated black-body radiation. At these high
temperatures an inert gas is used to sustain sonoluminescence. The
waveguide provides the means to extract a specified band of
wavelengths, collect the radiation from the at least one defect
cavity (typically multiple defect cavities) and efficiently route
the collected radiation to an output for emission. The waveguide is
on a much smaller scale than the acoustic crystal to avoid
interfering with the coupling of the sound waves to the defect
cavities. The waveguide may, for example, be a photonic crystal
defect waveguide, a photonic crystal optical fiber or a Sommerfeld
waveguide. Multiple waveguides tuned to different source
wavelengths may be coupled to a single acoustic crystal
sonoluminescent device. The marriage of the sonoluminescence
phenomena with an acoustic crystal provides for an efficient source
of narrow or broad band IR or THz radiation
[0018] These and other features and advantages of the invention
will be apparent to those skilled in the art from the following
detailed description of preferred embodiments, taken together with
the accompanying drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1, as described above, is a plot of bubble radius
versus time illustrating the expansion and collapse of gas bubbles
under a driving pressure produced by sound waves to produce
sonoluminescence;
[0020] FIG. 2, as described above, is a diagram illustrating the
formation, expansion and collapse of a gas bubble to emit light
through sonoluminescence;
[0021] FIGS. 3a and 3b are respectively a diagram of an acoustic
crystal sonoluminescent cavitation device and a defect cavity
therein in accordance with the invention;
[0022] FIG. 4 is a diagram illustrating the overlap of the
frequency spectra of the sound waves with the resonant frequency of
the acoustic crystal's defect cavity to concentrate energy into the
defect cavity to enhance sonoluminescence;
[0023] FIG. 5 is a plot of black-body curves at elevated
temperatures achievable using sonoluminescence;
[0024] FIGS. 6a and 6b are respectively a diagram of an acoustic
crystal sonoluminescent source of electromagnetic radiation and a
diagram of an embedded photonic crystal defect waveguide embedded
in the acoustic crystal to extract, collect and route a band of EM
radiation around a specified source wavelength to an output
antenna.
[0025] FIGS. 7a through 7c are respectively a black-body curve
illustrating the IR or THz bands emitted from the source, the
sequence of IR or THz pulses emitted and the extraction and
collection of black-body radiation over a volume dictated by device
geometry and the emission of the IR or THz band into a volume
determined by the antenna;
[0026] FIGS. 8a, 8b and 8c are respectively a plan view of embedded
photonic crystal optical fibers or Sommerfeld waveguides embedded
near defect cavities in the acoustic crystal to extract, collect
and route a band of EM radiation around a specified source
wavelength to an output antenna; and
[0027] FIGS. 9a and 9b are plan and end views of photonic crystal
optical fibers or Sommerfeld waveguides coupled to defect cavities
in the acoustic crystal.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The present invention describes an acoustic crystal
structure for enhancing sonoluminescence. Defect cavities in the
crystal structure concentrate the driving pressure from applied
sound waves into the cavity to cavitate gas bubbles in a liquid in
the defect cavity to produce sonoluminescence. A waveguide may be
coupled to the acoustic crystal to extract, collect and route a
band of electromagnetic (EM) radiation around a specified source
wavelength to an output port for emission by an antenna to provide
an EM source. The marriage of the sonoluminescence phenomenon with
an acoustic crystal provides for an efficient source of narrow or
broad band IR (approximately 1 micron to 30 micron) or THz
(approximately 300 GHz to 4 THz) radiation. By convention IR
sources are referred to in terms of wavelength while THz sources
are referred to in terms of frequency.
[0029] As used herein a `sound wave` refers to a pressure
fluctuation that travels through a medium (solid, liquid or gas) at
or near the speed of sound. A `shock wave` is a sound wave that is
traveling faster than the speed of sound in the medium. Shock waves
are typically characterized by an abrupt, nearly discontinuous
change in the characteristics of the medium. Across a shock there
is an extremely rapid rise in pressure, temperature and density of
the flow.
Acoustic Crystal Sonoluminescent Cavitation Devices
[0030] An acoustic crystal sonoluminescent cavitation device 50
includes an acoustic crystal 52 including at least one defect
cavity 54 containing a liquid 56 having a gas 58 dissolved therein
to form one or more bubbles 60 and a source 62 of sound saves such
as a piezo electric transducer (PZT) that provide the driving
pressure to cavitate the bubbles 60 to produce
sonoluminescence.
[0031] Acoustic crystal 52 includes a material 63 having a first
acoustic index and a material 64 having a second acoustic index
different than said first acoustic index. The materials may be a
solid, liquid or gas and may. The materials are arranged in a
periodic array that provides local contrast modulation of the
acoustic index in at least one dimension. A 2-D array as shown
provides modulation in 1-D. A 3-D array would provide modulation in
2-D. A local contrast modulation of at least 1.6 for a 2-D array
and 2.0 for a 3-D array creates a `band gap` 68 in the acoustic
transmission spectrum 70 as shown in FIG. 4. The position of the
band gap must be consistent with bubble formation and cavitation.
The wavelength at the center of the band gap is approximately equal
to or at least on the order of the spacing `d` in the periodic
array. Outside the band gap the energy in a sound wave
operationally coupled to the periodic array will be transmitted
through and partially absorbed by the crystal. Inside the band gap
the energy in the sound wave will constructively interfere and be
largely reflected. The more rows or layers to the periodic array
the better defined the band gap 68 in the acoustic transmission
spectrum 70.
[0032] The `acoustic index` is defined as the ratio of the speed of
sound in a control medium to the speed of sound in the material of
interest. We have selected diamond as the control medium although
any medium can be used. When computing the contrast or local
modulation of the acoustic index the control medium cancels out
leaving only the properties of the explosive materials and medium.
Table 1 lists a number of materials, the speed of sound in the
material and acoustic indices. As depicted there are many
combinations of materials that provide a local contrast modulation
(index1/index2) of greater than 1.5 or greater than 2.0.
TABLE-US-00001 TABLE 1 Material m/sec Acoustic Index Diamond 12000
1.00 Air (STP) 343 35 Aluminum 4877 2.46 Brass 3475 3.45 Copper
3901 3.08 Iron 5130 3.08 Lead 1158 10.36 Steel 6100 1.97 Water 1433
8.37
[0033] Defect cavity 54 in the periodic array creates a
transmission resonance 74 within band gap 68. In general a defect
may be any significant disturbance or "defect" in the periodic
structure e.g. the absence of material, different geometry of the
same material or a different material. However, defect cavity 54
must be some form of hole or void in the crystal structure in order
to support cavitation of a gas bubble in liquid inside the defect
cavity. The defect may be spherical or cylindrical for example. The
defect cavity may have a diameter of typically 0.5 mm to 2 cm to
sustain cavitation of, for example, bubbles having a 50 micron
diameter. In a cylindrical defect a line of bubbles may form along
the long axis of the cylinder thereby enhancing the total emitted
flux. Techniques to construct high-Q defect cavities are
well-known. The "Q" indicates how well the defect cavity resonates
over many cycles of the acoustic wave to concentrate and reach a
non-linear effect to initiate cavitation. Graded cavities are known
to provide high Q.
[0034] Liquid 56 is selected to allow for dissolution of the gas,
to provide the acoustic index contrast, to provide sufficient
elasticity to sustain cavitation at the frequency of the driving
pressure, and in certain cases to permit transmission of a portion
of the electromagnetic spectrum resulting from the
sonoluminescence. Liquids used in sonoluminescence have included
silicon pump oil, sulfuric acid, acetone or water
[0035] Gas 58 is selected to dissolve into the liquid and form
bubbles 60 that can be driven to expand and collapse. These bubbles
are typically on the order of a few microns in diameter although
they vary greater during the formation and collapse process.
Depending upon the application the gas may be one that undergoes a
chemical reaction when heated by the cavitation process. Gases such
as methane may be used. Alternately, the gas may be inert, suitably
one of the lighter noble gases such as Argon or Helium.
[0036] Source 62 produces sound waves 76 that are operatively
coupled to the acoustic crystal to produce a driving pressure. The
acoustic crystal and defect cavity are configured and the source of
sound waves selected so that the frequency content 78 of the sound
waves preferably overlaps band gap 68 and particularly resonance
74. For the most efficient transfer of energy from the sound waves
into the defect cavity, the dominant frequency 80 is aligned with
resonance 74. The defect cavity will concentrate energy from the
sound wave inside the defect cavity for some number of cycles. The
effect being to cavitate gas bubble(s) 60 and produce a short pulse
of high-temperature black-body radiation 82. This sonoluminescent
device may be used to research the sonoluminescent phenomena itself
or study cavitation or as a cavity to perform sonochemistry,
nuclear fusion, create nano-particles, chemical detoxification,
ultrasonic cleaning etc. For example, the pulse of black-body
radiation may be used to induce or enhance a desired chemical
reaction in the surrounding liquid in the defect cavity.
[0037] In general, the band gap may be positioned in the "acoustic"
band from 20 Hz to 20 kHz or the "ultra sonic" band form 20 kHz to
100 MHz. This spacing may range from as small as approximately 1
micron to as large as approximately 10 cm depending upon the
acoustic frequency, the materials in the periodic array, and the
position of the band gap. In a specific device configuration, the
band gap is positioned to balance competing interests such as size,
ease of manufacture, cost, ability to form and cavitate bubbles,
and in some cases the predetermined frequency of available acoustic
energy. For example, if the frequency of the sound waves is
relatively low than the spacing "d" of acoustic crystal, hence the
overall size of the device is relatively large. Also, the frequency
of the driving pressure must be high enough to form and cavitate
the bubbles yet not exceed the elastic properties of the liquid.
Consequently, the frequency of the sound waves may typically lie in
the ultra sonic band from approximately 20 kHz to 500 kHz with
typically spacing from 6 cm to 100 microns depending on the
frequency, materials, bubble sizes and reasonable device sizes.
[0038] In an embodiment, a PZT produces sound waves at 100 kHz. The
acoustic crystal is formed from a water medium with solid rods or
spheres periodically spaced through the medium. Any solid material
should provide sufficient modulation of the acoustic index. The
rods or spheres are held in place by, for example, a very thin
nylon or other suitable fiber network. The defect cavities are
provided by removing a rod/sphere from the periodic array and
allowing water to fill the void. An inert gas such as Ar is
dissolved in the water medium. Note, the pressure will only build
up to levels necessary for cavitation in the defect cavities, not
throughout the water medium. For a 100 kHz source, the wavelength
in water is approximately 1.5 cm. The rods/spheres are spaced
approximately 1.5 cm apart to create a band gap that overlaps 100
kHz with a resonance frequency approximately tuned to 100 kHz
[0039] In another embodiment, a PZT produces sound waves at 100
kHz. The acoustic crystal is formed from a solid silicon medium
with solid rods or spheres periodically spaced through the medium.
The rods/spheres may be formed from lead, copper, brass or other
suitable materials that provide the requisite modulation of the
acoustic index. The defect cavities are cylindrical or spherical
water filled holes in place of the lead filled rods or spheres. An
inert gas such as Ar is dissolved in the water filled holes. For a
100 kHz source, the wavelength in silicon is approximately 6 cm.
The rods/spheres are spaced approximately 6 cm apart to create a
band gap that overlaps 100 kHz with a resonance frequency
approximately tuned to 100 kHz.
[0040] Sonoluminescence has been demonstrated to produce
temperatures in excess of 15,000 K and up to 30,000 K. As shown in
FIG. 5, the curve 90 of black-body radiation shifts up and to
towards short wavelengths as the temperature increases. As shown in
the log-log plot of flux versus wavelength, the increase in flux
across the spectrum from 300K (room temperature) to 30,000K is near
three orders of magnitude (1000.times.) in the THz region 92 and
considerably larger in the IR region 94.
[0041] Although the sonoluminescence phenomena produces the high
temperature pulse, the use of an acoustic crystal with defect
cavities to concentrate the sound energy to cavitate the bubbles
should improve the power efficiency and may increase the associated
temperature and/or the number of bubbles cavitated, either of which
increase the radiated flux. An additional benefit is that a single
source 62 may be used to provide sufficient driving pressure to
cavitate the bubbles inside multiple defect cavities. Typical
sonoluminescent devices require multiple PZTs.
Acoustic Crystal Sonoluminescent Electromagnetic Sources
[0042] THz-frequency radiation, in the frequency region from
approximately 300 GHz to approximately 4 THz, has been relatively
unexploited compared to the adjacent radio frequency (RF) and IR
spectral bands. This is largely because of transmission
difficulties due to absorption by atmospheric water vapor but also
due to a lack of practical radiation sources. In recent years there
has been a significant growth of interest in applications of this
previously underutilized portion of the electromagnetic
spectrum.
[0043] One of the major bottlenecks for the successful
implementation of THz-frequency systems is the limited output power
of conventional THz sources. Most systems produce THz radiation via
optical techniques, but those require massive lasers, complex
optical networks and cooling systems. Some of the THz sources
reported in the literature include optically pumped THz lasers,
time-domain spectroscopy, backward wave oscillators, solid-state
amplifiers combined with direct multipliers, and photo-mixers
(Iida, M. et al., Enhanced generation of terahertz radiation using
3D photonic crystals with a planar defect, Proc. CLEO/QELS, June
2002 (Baltimore), Section CM1; Unterrainer, K. et al.; Cavity
enhanced few cycle THz generation and coherent spectroscopy, Proc.
CLEO/QELS, June 2002 (Baltimore), Section CM1; Han, H., Park, H.,
Cho, M., and Kim, J., Terahertz pulse propagation in a plastic
photonic crystal fiber, Applied Physics Lett., 80 #15, 15 Apr.
2002). The different sources have disadvantages including limited
output power; excessive cost, size and weight; poor reliability and
limited frequency agility. U.S. Pat. Nos. 7,078,697 and 7,257,333
are directed to the use of photonic crystals to produce enhanced
thermal emissions in the IR and THz regions.
[0044] As discussed, the cavitation of bubbles produces short
pulses of very high temperature black-body radiation. With known
sonoluminescent devices, this radiation is seen as short flashes of
blue light. The configuration of those devices is such that the
wavelengths for blue light can escape and be seen or measured. The
other wavelengths of the visible spectrum as well as the longer IR
and THz spectrum are readily absorbed in the liquid inside the
cavity and lost.
[0045] The acoustic crystal sonoluminescent cavitation device may
be operatively coupled to a waveguide tuned to a specified center
wavelength to extract a band of wavelengths around the center
wavelength from the enhanced black-body radiation, collect the
radiation from the one or more defect cavities in the acoustic
crystal and route the radiation to an output port for controlled
emission. By positioning the waveguide close to the defect cavity
(e.g. suitably less than 5.times. the extracted IR or THz
wavelength) the desired wavelengths can be efficiently extracted.
Various technologies such as embedded photonic crystal defect
cavities, photonic crystal optical fibers and Sommerfeld waveguides
may be used in 2D or 3D configurations to extract the specified
bands. The IR and THz bands are of particular interest due to the
general lack of commercially viable sources. The "Q" indicates how
well the defect cavity resonates over many cycles of the acoustic
wave to concentrate and reach a non-linear effect to initiate
detonation and thus determine the width of the band. Techniques to
construct high-Q defect cavities are well-known. Graded cavities
are known to provide high Q. The waveguide could, however, be
configured to extract any portion of the black-body radiation.
[0046] An embodiment of an acoustic crystal sonoluminescent source
100 that combines an acoustic crystal cavitation device 102 and an
embedded photonic crystal defect waveguide 104 is illustrated in
FIGS. 6a and 6b. Sonoluminescent cavitation device 100 comprises an
acoustic crystal 106 including first and second materials 108, 110
having difference acoustic indices. The materials are arranged in a
periodic array that provides local contrast modulation of the
acoustic index in at least one dimension to define a band gap in
the acoustic or ultrasonic transmission spectra. At least one and
as shown multiple defect cavities 112 in the periodic array create
a resonance in the band gap. An inert gas is dissolved in a liquid
in the defect cavities. A source 114 of sound waves 116 is
operatively coupled to the periodic array. The sound waves are
concentrated in the defect cavities to cause the bubble to expand
and collapse (cavitate) producing a pulse of broadband black-body
radiation.
[0047] Photonic crystal defect waveguide 104 includes third and
fourth materials 120, 122 having different refractive indices. The
materials are arranged in a periodic array that provides local
contrast modulation of the refractive index in at least one
dimension to define a band gap in the electromagnetic spectrum
(e.g. within the IR or THz bands). The photonic crystal has a
period at least 10.times. smaller than the period of the acoustic
crystal. As such the embedded waveguide does not interfere with the
sound waves propagating through the acoustic crystal. At least one
and typically multiple defects 124 in the periodic array of the
photonic crystal are tuned to resonate at the specified source
wavelength. The defects are positioned to couple to the acoustic
defect cavities 112 and to each other to extract radiation 126 in a
band around the source wavelength emanating from the acoustic
defect cavities, collect the radiation and routing it to an output
port for emission by an antenna 128. For example, defects 124 are
suitably positioned within 5.times. the extracted source wavelength
and more preferably within 3.times. from the acoustic defect cavity
112 and each other. In this embodiment, antenna 128 is a horn
formed from the photonic crystal itself.
[0048] The `refractive index` is defined as the ratio of the speed
of light in a vacuum to the speed of light in the material of
interest. Table 2 lists a number of materials with their refractive
index. As depicted there are many combinations of materials that
provide a local contrast modulation (index1/index2) of greater than
1.5 or greater than 2.0.
TABLE-US-00002 TABLE 2 Material Refractive Index Benzene 1.501
Water 1.333 Ethyl Alcohol 1.361 Carbon Tetrachloride 1.461 Carbon
Disulfide 1.628 Diamond 2.419 Strontium Titanate 2.41 Amber 1.55
Fused Silica 1.458 Sodium Chloride 1.50 Cubic Zirconia 2.15-2.18
Moissanite 2.69-3.02 Gallium (III) Phosphide 3.5 Gallium (III)
Arsenide 3.927 Silicon 4.01
[0049] In an embodiment, the photonic crystal defect waveguide 104
is configured for use with an acoustic crystal cavitation device
102 in which solid rods or spheres are periodically spaced in a
water medium and driven at 100 kHz as previously described. The
defect cavities being absent rods or spheres in the periodic array.
The photonic crystal waveguide is relatively small, typically 1 to
3 mm diameter or square, silicon rods 120 with holes 122 spaced at
1 to 500 micron intervals with missing holes (solid silicon)
forming the defects 124. As such the waveguide does not interfere
with the coupling of the incident sound waves to the acoustic
defect cavities. The spacing of holes 122 determines the band gap
and resonance of the photonic crystal. IR is approximately 1 to 30
microns and THz is typically 300 GHz to 4 THz. The "Q" of the
defect will determine the width of the extracted band. For IR,
typical bandwidths range from 0.1 to 3 microns. For THz, typical
bandwidths range from 30 GHz to 400 GHz.
[0050] In another embodiment, the photonic crystal defect waveguide
104 is configured for use with an acoustic crystal cavitation
device 102 in which solid rods or spheres are periodically spaced
in a silicon medium and driven at 100 kHz as previously described.
The defect cavities being water filled cavities in the periodic
array. The photonic crystal waveguide can be integrated directly
into the silicon medium of the acoustic crystal near the defect
cavity elements. As above, the holes 122 are spaced from 1 to 500
microns depending on the desired source wavelengths with missing
holes providing the defects.
[0051] As shown in FIGS. 7a through 7c, acoustic crystal
sonoluminescent source 100 emits narrow or broadband radiation 126
in the IR or THz bands 130, 132, respectively in a sequence of
pulses 134 with a prescribed cone 136. As described previously the
center wavelength and width of the band are set by the design of
the photonic crystal and defects. The total flux radiated is a
function of not only the temperature produced by the cavitation of
an individual bubble (which may be enhanced by the concentration of
sound energy into the defect cavity) but also the total number of
bubbles in each cavity (may be enhanced by the concentration of
energy), the total number of acoustic defect cavities that are
coupled together and the low loss of the photonic crystal defect
waveguide to extract and route the radiation to the antenna. As
shown in FIG. 7b, the repetition rate of pulses 134 may equal to
that of the sound waves (assuming each cycle produces cavitation)
or a multiple thereof (assuming N cycles produce 1 cavitation). As
shown in FIG. 7c, the black-body radiation 126 emanates in all
directions from a collapsing bubble. The waveguide may be
configured to extract and collect the band of radiation in all
directions and route the accumulated radiation to the antenna. The
antenna can be configured to concentrate the energy and emit the
band of radiation in a specified cone as opposed to radiating in
all directions. As such the combination of the acoustic crystal
cavitation device with a waveguide may produce viable IR or THz
sources.
[0052] Another embodiment of an acoustic crystal sonoluminescent
source 200 that combines an acoustic crystal cavitation device 202
and an embedded waveguide 204 is illustrated in FIGS. 8a through
8c. Sonoluminescent cavitation device 200 comprises an acoustic
crystal 206 including first and second materials 208, 210 having
difference acoustic indices. The materials are arranged in a
periodic array that provides local contrast modulation of the
acoustic index in at least one dimension to define a band gap in
the acoustic or ultrasonic transmission spectra. At least one and
as shown multiple defect cavities 212 in the periodic array create
a resonance in the band gap. An inert gas is dissolved in a liquid
in the defect cavities. In one configuration, the defect cavities
are a 2D array of cylinders 214 as shown in FIG. 8b. In another
configuration, the defect cavities are a 3D array of spheres 216 as
shown in FIG. 8c. A source 218 of sound waves 220 is operatively
coupled to the periodic array. The sound waves are concentrated in
the defect cavities to cause the bubble 222 to expand and collapse
(cavitate) producing a pulse of broadband black-body radiation
224.
[0053] Waveguide 204 may be, for example, a photonic crystal
optical fiber or a Sommerfeld waveguide (uncoated wire). Multiple
waveguides tuned to different source wavelengths may be embedded
around the defect cavities or possibly inside the defect cavities.
The spacing between the acoustic defect and the waveguide depends
on wavelength extracted; longer wavelength THz must be closer than
IR. The fibers or wires are collected into a common fiber or wire
that routes the extracted radiation to an output port and antenna
(not shown). Photonic crystal optical fibers typically include a
hollow core (defect) surrounded by a pattern of small holes in the
fiber to form a 2D photonic crystal that constrains the band of
electromagnetic energy that is coupled into the fiber and travels
along the interior of the fiber without losses. The far end of the
fiber is suitably terminated with a mirror. Such fibers are used at
1.5 micron in the near IR for telecommunications and at 10.6
microns in the LWIR for medical applications. Johnathan C. Knight's
article on "Photonic Crystal Fibres" published in Nature, VOl 424,
Aug. 14, 2000 pp. 847-851 provides a more complete description and
is hereby incorporated by reference. Sommerfeld waveguides
typically include an uncoated wire that supports surface-wave
propagation for THz bands. The radius of the wire is set to select
the source wavelength. Sommerfeld waveguides are well known and
described by M. J. King and J. C. Wiltse in "Surface-Wave
Propagation on Coated or Uncoated Metal Wires at Millimeter
Wavelengths" published in IRE Transaction on Antennas and
Propagation May 1962 pp. 246-254, and Righini et al. in "Terahertz
Flexible Waveguides" published in Proceedings of SPIE, Vol. 7366,
p. 776660Z-1 (June 2009), which are hereby incorporated by
reference.
[0054] Another embodiment of an acoustic crystal sonoluminescent
source 300 that combines an acoustic crystal cavitation device 302
and a waveguide 304 is illustrated in FIGS. 9a and 9b. In this
embodiment, the waveguides 304 (either photonic crystal optical
fibers or Sommerfeld waveguides) are positioned above the acoustic
defect cavities 306. Each wavegfuide may extract radiation in the
desired band from one or more acoustic defect cavities. The far end
of the optical fiber is suitably terminated with a mirror 307. A
common waveguide 308 collects radiation from each of the waveguides
and routes the radiation to an output port 310. This structure may
be stacked into a 3D structure and the outputs coupled together to
further enhance the emission of radiation.
[0055] While several illustrative embodiments of the invention have
been shown and described, numerous variations and alternate
embodiments will occur to those skilled in the art. Such variations
and alternate embodiments are contemplated, and can be made without
departing from the spirit and scope of the invention as defined in
the appended claims.
* * * * *